In the plating of dielectric (non-conductor) substrates by chemical (electroless) plating, it is well known that suitable catalytic treatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic pretreatment, it appears that while different procedures have been used, the incorporation of precious metals (e.g., palladium-containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. In the process disclosed, a colloidal solution composed of tin(II) and precious metal salts, generally with hydrochloric acid, is used. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g., palladium) stabilized by the excess stannous chloride present in the medium. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metals and miscellaneous product reliability problems have led to the quest for new systems in which the use of precious metals, tin, as well as that of the hydrochloric acid would be completely eliminated.
In meeting this objective it was found, as described in U.S. Pat. Nos. 3,993,799 and 4,087,586 filed by the applicant herein, that colloidal systems (metals, alloys, and compounds) based upon non-precious metals could constitute the basis for new commercial plating processes. It was found that colloids of non-precious metals (preferably selected from, but not limited to, the group of copper, iron, cobalt, nickel and combinations thereof) may be used in the replacement of the tin/palladium colloid followed by a treatment in a suitable reducing medium. In the reducing medium, reduction of the ionic portion of adduct (or surface of colloid) derived through the adsorption from the colloidal medium takes place, resulting in active nucleating sites capable of initiation of the electroless process. It is noted, however, that the reducing medium can be deleted since most electroless plating baths contain at least one reducing agent. Accordingly, the present invention is applicable to both modes of processing.
In reviewing the teaching disclosed in U.S. Pat. No. 3,993,799, it is recognized that many of the inherent disadvantages associated with the palladium based catalysts are eliminated. It is further recognized that based upon practices in this art, it is essential that any catalytic system should maintain its properties especially with storage (e.g., several months) and shipment under conditions of substantial temperature fluctuation, as well as maintaining stability during usage. It is thus highly desirable to have a medium in which good colloidal stability would be maintained, while at the same time having sufficient catalytic activity to be used in the plating process. I have generally observed that as one increases stability, the activity for plating initiation is decreased thereby making it difficult to meet both requirements in a single system.
For example, I have observed that active plating colloids have generally shown a limited stability (for long term storage purposes) due to coagulation which takes place leading to precipitation, with change in particle size distribution during the coagulation process. In addition, I have noted that highly stable colloidal dispersions have shown limited catalytic activity when used in accordance with U.S. Pat. No. 3,993,799 and moderate concentrations of reducing medium or activating medium. Similar trends were also noted in U.S. Pat. No. 3,948,048 on the interrelationship between reactivity and stability. In fact, in U.S. Pat. No. 3,958,048 some of the illustrated examples lost their colloidal character and became true solutions within 24 hours of aging.
It is thus an objective of this invention to provide both stable and active colloidal dispersions which are useful in electroless plating processes, as well as novel methods for their stabilization. While not wishing to be repetitious, the following are included herein by reference: U.S. Pat. Nos. 3,011,920; 3,993,799; 3,958,048; 3,993,491; 3,993,801; 4,087,586; 4,180,600; 4,131,699; 4,167,596; 4,151,311; 4,132,832; 4,136,216; 4,261,747; 4,259,376; 4,258,087; 4,228,201; 4,224,178; 4,220,678; 4,228,201; 4,181,760; and Ser. No. 061,484.